US 20110088258 A1
A high-power, shielded, single-pole electrical connector and method for installing such a connector are disclosed. The connector has a single-pole connector housed within an electrically conductive outer shell. The inner, single-pole connector is electrically insulated from the outer shell. A shielding trap is used to provide electrical contact between the outer shell of the connector and a shielding layer of a shielded electrical supply cable. The inner, single-pole connector may be a male-female type or a lug-type. If a lug-type single-pole connector is used, a dual-shell, cylindrical insulator may be used to provide access to the lug bolts. Such an insulator may be realigned after the lug-bolt access is no longer required so that a complete insulating barrier is provided around the lug-type connector. A variable-angle, lug-type connector may be used.
1. A method of connecting a shielded, single-pole electrical connector to a high-power, shielded electrical supply cable comprising:
a. stripping the supply cable to expose its layers as follows:
i. approximately 1.5 to 1.75 inches of a core conductor;
ii. approximately 0.75 to 1.25 inches of a core conductor insulation; and,
iii. approximately 0.25 to 0.75 inches of a shielding layer;
b. connecting a high-power, single-pole electrical connector to the exposed portion of the core conductor;
c. connecting a shielding trap to the exposed portion of the shielding layer, such that the core conductor insulation is positioned between the shielding trap and the high-power, single-pole electrical connector;
d. positioning an insulating barrier around at least a portion of the high-power, single-pole electrical connector; and,
e. positioning an electrically conductive outer shell over the insulating barrier, the high-power, single-pole electrical connector, and the shielding trap such that the shielding trap is in electrical contact with the outer shell and the outer shell is electrically isolated from the high-power, single-pole electrical connector.
2. The method of
3. A method of connecting a shielding trap to a shielding layer of a high-power, shielded electrical cable comprising;
a. stripping the cable to expose approximately 0.2 to 0.6 inches of the shielding layer;
b. positioning a first cylindrical ring of the shielding trap over the cable such that the first cylindrical ring is positioned on the unstripped cable side of the exposed shielding layer;
c. lifting the shielding layer away from the longitudinal axis of the cable so that the shielding layer is positioned at an angle of approximately 30° to 60° from the longitudinal axis of the cable;
d. positioning a second cylindrical ring of the shielding trap on the side of the exposed and lifted shielding layer nearer the stripped end of the cable, such that the shielding layer is positioned between the first and second cylindrical rings; and,
e. securing the first and second cylindrical rings together such that the exposed and lifted shielding layer is secured between the two rings, providing a secure physical and electrical connection between the shielding layer and the shielding trap.
4. The method of claim 17, wherein the first and second cylindrical rings are screwed together in step e.
This application is a divisional of U.S. patent application Ser. No. 12/151,099, filed May 2, 2008.
The invention relates to a shielded, single-pole electrical connector for use in high-power applications. The invention is particularly suited for use with high-power variable frequency AC drive motors.
Pending patent application Ser. No. 12/015,661, which is co-owned with the current application, is hereby incorporated by reference.
AC motors spin at a speed determined by the number of poles and the frequency of the applied AC current. The speed in revolutions per minute (RPM) is equal to 120 × frequency (Hz) divided by the number of poles. For example, a motor with four poles operating at 60 Hz, would have a nominal speed of 1800 rpm. The operating speed of traditional AC motors is relatively constant, though in practice, the loaded speed does vary.
The rotational speed of DC motors, on the other hand, varies with supply voltage. By reversing the polarity of the supply voltage, a DC motor will reverse direction. Speed control, therefore, is a fairly simple matter with DC motors. When speed control is important, and the ability to reverse the direction of rotation is also needed, DC motors provide one effective option.
The oil industry is one area where high-power rotational motors with reliable speed control are used. An oil well is drilled by rotating a drill string with a drill bit at its end. Today, it is common for a variety of exploration tools to be mounting in the drill string, typically near the drill bit. These tools measure temperature, pressure, density of the formation, resistivity or conductivity of the formation, and various other parameters of interest to oilfield exploration engineers.
In an oilfield drilling operation, it is desirable to control the speed of the drill motor. This can be important for optimum effectiveness of the drilling bit, for removal of cuttings, and for the operation of tools installed in the drill string. Large DC motors traditionally have been used in the oilfield for this purpose. These motors are not very efficient, but they do provide reasonably good control of the rotational speed of the drill string. These motors also provide high torque, which is crucial in this industrial setting.
Variable frequency drive (VFD) AC motors have become increasingly popular in recent years, including in the oilfield industry. VFD motors are a good alternative to DC motors, in large part because the VFD motors are more efficient. Improvements in the technology in recent years have made large VFD motors a reliable, efficient option in many heavy industries. The oilfield industry has been opting for large VFD motors more and more in recent years.
To supply VFD motors, two conversions are done. First, the AC supply is converted to DC, and then the DC is converted to a variable frequency AC signal. In the most common arrangement, the variable frequency AC signal is made up of a series of DC pulses. Pulse width modulation of a DC output is used to create a simulated AC sine wave signal. The DC polarity is reversed to create the negative portion of the simulated sine wave.
This process involves a great deal of high speed switching. In high-power applications, the switching components may have to switch on and off thousands of times per second, and may rise and fall by hundreds of volts with each switch. This type of switching produces a great deal of harmonic and switching noise in the system. These noise components of the total signal will be carried by the conductors from the power supply to the motors.
The VFD noise can cause problems with electronic systems operated in the same physical area. Computer equipment can experience problems. Control and monitoring equipment also may experience problems due to the VFD noise. VFD motors offer important benefits, but the problems caused by the VFD noise must be controlled, or this problem may outweigh the benefits of a VFD system.
To limit the transmission of the noise signals, shielded power cables are typically used application were VFD noise poses a problem. Again, the oilfield industry provides a good example. During the oil drilling operation, computers and other electronic equipment are used to monitor and evaluate various parameters. VFD noise can cause serious problems in the oil drilling situation if it is not controlled. Shielded power cables are often used for this reason in oilfield applications where VFD motors are used.
A typical shielded cable application in the oilfield might involve use of single, shielded power cables running from the VFD power supply to the VFD motor. The cables are hard-wired at each end, so no separate inline connectors are used. The shielding is grounded at one or both ends of the run. The internal, shielded, power conductor supplies the VFD current to the VFD motor. The continuous run of shielding on the power cable contains most of the potentially harmful VFD noise.
This typical arrangement will not work, however, if a connection is needed somewhere between the supply and the drive motor, or at either end of the power cable. For example, if the run from the VFD power supply to the VFD motor is too long for a single cable, it is necessary to use some type of inline connector to piece together the different sections of shielded cable. This may be a fairly common situation because the shielded cable used in oilfield and other heavy industries tends to be quite large and heavy. Such cable may weigh several pounds per foot, making long cable runs quite heavy and unwieldy. Using shorter sections of cable connected together with separate connectors is one way of addressing this problem.
Cable connections also may be needed at the VFD motor or at the supply. Use of a connector at these points allows for easier replacement of a cable, when compared to a hard-wired arrangement. In oilfield drilling operations, the drive motor may be moved up and down during the drilling process. The drive motor may also need to be moved to another position for service or inspection. With so much movement, the connections between the cable and the drive motor will be subject to stress and may fail after extended use. In addition, if the drive motor is to be moved for inspection or service, there may be a need to disconnect the drive motor from its supply cables. These connection and disconnection operations are much easier to accomplish is a separate connector is used, as opposed to hard-wiring the supply cables to the drive motor.
If a nonshielded connector is used, some of the noise found in the VFD power lines will be transmitted to various items that may be damaged by such noise. Computers and other electronic equipment may be vulnerable to such damage. It is, therefore, highly desirable to ensure than the entire electrical path from the VFD power supply to the VFD motor, including all connections, is fully shielded. Shielded power cables are relatively easy to find, but there remains a need for high-power shielded connectors.
The need for an inline or end-of-cable connector in high-power VFD applications poses a problem. Low power shielded cable connectors are well known. Such connectors are used widely on home cable television and Internet systems. The small, shielded connectors used in such applications provide a continuous shield for any noise that exists on the cable signal.
In a typical low power shielded connector, the cable has a small internal core conductor that carried the signal of interest. An insulator surrounds the core conductor, and a braided shield surrounds the core insulator. Another insulator, typically the outer covering of the cable, is positioned over the braided shield wires. The shielded connector connects the braided shield wires to the outer shell of the connector and connects the core conductors while providing an insulation layer between the core conductors and the shell of the connector. In this manner, a continuous electrical path is provided for both the core conductor and the braided shield, with these two paths being electrically insulated from each other.
The same concept is possible, and needed, in the high-power VFD motor context. It is, however, a huge step to go from the small, shielded connectors used with home cable television systems to the sort of shielded connector needed for a high-power VFD situation. The core conductor in a home television cable is not much larger than a piece of thread or fishing line. The cable is light, the shielding is very thin and easily handled. The current capacity of these systems, and the connectors used with these systems, is quite low.
In an oilfield VFD application, on the other hand, the cables can weigh hundreds of pounds. The core power conductors can be an inch thick or more and are very stiff. The shielding used in these high-power applications is much heavier and harder to work with than the thin shielding braid found on a home television cable. Cutting, crimping, and other typical tasks associated with making up electrical connectors all take on a very different nature when large, high-power cables are involved.
One particular challenge found in the high-power VFD application that is not present with low power cable television connectors is the difficulty in making up nearly identical connections repeatedly. Given the size, weight, and stiffness of the large power cables involved in high-power VFD applications, it is not practical to use a connector that requires precise and consistent positioning of all the connections between the connector and the supply cable. It is, therefore, highly desirable for a high-power VFD connector to allow for some variance in the positioning of the connections involved, while still providing a reliable, fully shielded connector.
Because the supply cable used in high-power VFD applications is so heavy and stiff, it is almost impossible to make up a connection with such cable if a quick turn or change of direction is required. Consider, for example, a connection made in a physical space where the supply cable must turn 45° immediately after the point of connection. It may not be possible to bend the cable to create this sharp a turn. There is a need, therefore, for a connector that solves this problem by allowing for use of heavy, shielded power cables, while providing the ability to make sharp bends or turns.
Finally, it is desirable for this connector to have an internal insulator between the shielded shell of the connector and the internal power conductor. Such an insulator should allow for access to lug bolts while also providing the capability to fully isolate, electrically, the internal power conductor once the connection has been made up. The insulator should be reliable and easy to use.
The present invention provides the high-power shielded connector needed for use with high-power VFD motors and power supplies. In a preferred embodiment, the connector includes a high-power, single-pole electrical connector; an electrically conductive, generally cylindrical outer shell having an internal electrical contact region; an electrically insulating layer positioned between the single-pole connector and the electrically conductive outer shell; and, a generally cylindrical shielding trap configured to provide an electrical connection between a shielding material of a high-power, electrical cable and the internal electrical contact region of the electrically conductive outer shell.
The method of connecting a high-power, shielded electrical cable to the connector includes stripping the supply cable to expose its layers as follows: approximately 1.5 to 1.75 inches of a core conductor; approximately 0.75 to 1.25 inches of a core conductor insulation; and, approximately 0.25 to 0.75 inches of a shielding layer. A high-power, single-pole electrical connector is connected to the exposed portion of the core conductor. A shielding trap is connected to the exposed portion of the shielding layer, such that the core conductor insulation is positioned between the shielding trap and the high-power, single-pole electrical connector. An insulating barrier is positioned around at least a portion of the high-power, single-pole electrical connector; and, an electrically conductive outer shell is positioned over the insulating barrier, the high-power, single-pole electrical connector, and the shielding trap such that the shielding trap is in electrical contact with the outer shell and the outer shell is electrically isolated from the high-power, single-pole electrical connector.
The present invention is best understood through reference to the accompanying drawings.
The parts of the cable 12 are shown in more detail in
The next layer of the cable 12 is the core conductor insulation 16. This is a solid layer of electrically insulating material surrounding the core conductor 14. In the high-power applications, the insulator material must be chosen from a stable material that is not subject to breakdown at relatively high operating temperatures. With such high currents possible in the core conductor 14, considerable heat may be generated during use. The core conductor insulator 16 must be capable of withstanding high temperatures without breaking down.
The shielding 18 is the next layer of the cable 12. In high-power operations, the shielding is relatively heavy and stiff. Shielding may be braided or a solid layer, though braided shielding is believed to be more common. Either type works with the present invention. Some high-power shielded cables include a thin layer of Mylar or other similar material around the core conductor insulation 16. This type of configuration is not shown, and its use or nonuse is not material to the present invention. The shielding 18 is covered by the outer insulation 20. Another outer layer of highly durable material may be placed around the outer insulation 20, though the use of such material does not impact the performance of the present invention. Such materials, however, may be desirable to prevent excessive wear to the power cables in environments where such cables are subjected to considerable stress and wear.
When used with a preferred embodiment of the present invention, the core conductor 14 is stripped so that approximately 1⅝″ of the conductor are bare. The core conductor insulation 16 is striped so that approximately 1″ of it is exposed. About ½″ of the shielding is exposed by the stripping process. This produces the “stair-step” cross-section shown in
On the right side of
The conceptual drawing illustrates the key operational characteristics of the invention. Two distinct electrical paths are maintained through the connector, with the core conductor path being fully contained within the outer shielding path. Thus, the core conductor is fully surrounded by an electrical shield both in the cable 12 and in the connector 10. This is the key feature needed by a high-power, shielded connector. The connector 10 must provide a reliable, low-resistance electrical connection for high power lines that is fully shielded.
Given the size and stiffness of the high-power cables, it is difficult to make a shielded connector that is both functional (i.e., meets the functional needs described above) and user friendly. For example, if a shielded connector is designed so that the power cable connections must be made to precise tolerances, the connector will be of little use in the field.
The nature of the connection between the shielding 18 and the conductive outer shell 22 poses another challenge. The connector cannot be too large in diameter or it will be too unwieldy to be of practical use in the field. The power cables used in these high-power applications typically range from 1.5 to 2.5 inches in outside diameter. The connector 10, therefore, should be approximately 3-4 inches in diameter, at the most. If the connector 10 is much larger than that, its size will make it less practical.
Given these sizing constraints, and the stiffness of the cable, it is difficult, if not impossible, to use a fixed or permanent connection between the shielding 18 and the outer shell 22 of the connector 10. Once a single pole conductor is connected to the core conductor 14, the cable 12 is positioned within the outer shell 22. At this point, the outer shell 22 covers the entire length of the exposed shielding 18. To make a fixed or permanent connection between the shielding 18 and the outer shell 22 would require an operator to somehow work within the small space between the cable 12 and the outer shell 22. Given the size and stiffness of the cable 12, such an operation is simply not feasible.
Nor is it feasible to use a fixed internal contact within the outer shell 22. If this were done, the operator would have to strip the cable 12 to precise length requirements and the operator would still have to make up at least part of the connection between the shielding 18 and the outer shell 22 within the small space between the cable 12 and the shell 22.
The present invention solves these challenges by using a shielding trap 30, which is illustrated in
A first threaded ring 32 and a second threaded ring 34 are shown in
A fully made-up shielding trap 30 is shown in
In order to provide a good electrical connection between the shielding trap 30 and the outer shell 22, a contract region 46 is provided within the outer shell 22. This contact region 46 may be formed by machining away a very small layer of the shell 22, thus removing any coating material and exposing the more conductive material of the shell 22. The contact region 46 is sufficiently long to allow for some play in the positioning of the shielding trap 30. In a preferred embodiment, a contact region 46 of about ½ to ¾ inch is long enough to provide the needed play. A longer contact region 46 may be advantageous in some situations to provide ever greater tolerance for variations in the lengths of the stripped cable 12. This might be desirable when connectors are used in environments like the North Atlantic Sea or north of the Arctic Circle, where very low temperatures may make working with these types of materials even more difficult.
A cross-section of the connector 10 of the present invention is shown in
The method of assembling the connector can vary somewhat, but a preferred sequence follows. The cable 12 is stripped to leave the desired lengths of the various parts exposed. The shield trap 30 is then connected to the shielding 18. The female single pole connector 26 (as shown in
In the embodiment shown in
In practice, a male version of the connector shown in
The lug connector 52 is secured to another connector or a contact using a first lug bolt 56 and a second lug bolt 58. An oversized, tapered hole is provided to accommodate the head of the lug bolt. To reduce the length of the lug connector 52, the lug bolt holes can be configured so that one lug bolt is started from each side of the connector. That configuration is shown in
When the lug bolt arrangement described above is used, it is necessary to access both sides of the lug connector to tighten or loosen both lug bolts. It is also necessary to provide insulation between the lug connector 52 and the outer shell 22. This poses a challenge, because if an access hole is provided in the insulator, then a gap in the insulation would exist at the access hole.
To solve this problem, the present invention utilizes a dual sleeve insulator 60, which is shown in
Two oval access holes 66 are provided, one in each of the two cylindrical insulating shells. The two shells may be rotated relative to each other to align the access holes 66, as shown in
In operation, the insulator 60 is used as follows. The first cylindrical insulating shell 62 is rotated so that the oval access holes 66 are aligned with each other. Both insulating shells are then rotated together until the oval access holes 66 are positioned over one side of the lug bolts holes, as shown in
The lug connector 52 also allows for angled connection, as shown in
It is possible, however, that a sharp bend or turn in the power line path may be needed inside the motor casing 70. This can be accomplished using the lug connector 52 and only a first lug bolt 56. The connector 52 can be positioned at almost any angle when connected in this manner, which provides desirable space saving within the casing of a motor or other component. Variable angle connections of this type are not possible when using the full connector 10, having the outer shell 22, as described above, but variable angle connections are feasible inside the casings of motors or other components. This is desirable because the need for an angled connection may be most common within motors or other components, rather than for in-line connectors.
The connector of the present invention allows for relatively easy and reliable installation in the field. The method of installing the connector 10 includes stripping the high-power electrical cable 12 to expose its inner layers, as shown in
A high-power, single-pole electrical connector is connected to the exposed part of the core conductor. This can be done before or after the shielding trap 30 is connected, but it may be simpler to connect the shielding trap first because of the added weight of the single-pole connector. The sequence is not critical, however, unless the single-pole connector has a larger outside diameter than the inside diameter of the first and second threaded rings of the shielding trap 30. If that is true, the shielding trap rings must be installed before the single-pole connector is connected.
When the shielding trap 30 and single-pole connector have been securely connected to the shielding 18 and the core conductor 14, respectively, an insulating barrier may be positioned over part or all of the single-pole connector. The outer shell 22 may then be positioned over the other parts of the connector.
While the preceding description is intended to provide an understanding of the present invention, it is to be understood that the present invention is not limited to the disclosed embodiments. To the contrary, the present invention is intended to cover modifications and variations on the structure and methods described above and all other equivalent arrangements that are within the scope and spirit of the following claims.